The compound titanium nitride (TiN) has numerous potential applications because it is extremely hard, chemically inert (although it readily dissolves in hydrofluoric acid), an excellent conductor, possesses optical characteristics similar to those of gold, and has a melting point around 3000.degree. C. This durable material has long been used to gild inexpensive jewelry and other art objects. However, during the last ten to twelve years, important uses have been found for TiN in the field of integrated circuit manufacturing. Not only is TiN unaffected by integrated circuit processing temperatures and most reagents, it also functions as an excellent barrier against diffusion of dopants between semiconductor layers. In addition, TiN also makes excellent ohmic contact with other conductive layers.
Until little more than a year ago, reactive sputtering, the nitrogen anneal of an already deposited titanium layer, and high-temperature atmospheric pressure chemical vapor deposition (APCVD), were the three principal techniques available for creating thin titanium nitride films. Reactive sputtering and nitrogen anneal of deposited titanium result in films having poor step coverage, which are not useable in submicron processes. Chemical vapor deposition process have an important advantage in that a conformal layers of any thickness may
This is especially advantageous in ultra-large-scale-integration circuits, where minimum feature widths may be smaller than 0.5.mu.. Layers as thin as 10.ANG. may be readily produced using CVD. However, TiN coatings prepared used the high-temperature APCVD process must be prepared at temperatures between 900.degree.-1000.degree. C. using titanium tetrachloride, nitrogen and hydrogen as reactants. The high temperatures involved in this process are incompatible with conventional integrated circuit manufacturing processes. Hence, depositions using the APCVD process are restricted to refractory substrates such as tungsten carbide.
The prospects for the use of TiN films in integrated circuits improved in 1986, when Roy G. Gordon and Steven R. Kurtz, colleagues in the Department of Chemistry at Harvard University, announced at the Material Research Society Symposium that TiN could be deposited in a new APCVD process at lower temperatures (1986 Mat. Res. Soc. Symp. Proc. Vol. 140, p. 277). Using this process, titanium chloride is reacted with ammonia within a temperature range of 500.degree.-700.degree. C. However, even this temperature is incompatible with silicon chips metallized with aluminum, amorphous silicon solar cells and plastics. In addition, the presence of hydrogen chloride gas, a corrosive by-product of the reaction, is undesirable.
Some thirty years ago, D. C. Bradley and I. M. Thomas showed that, in solution, Ti(NR.sub.2).sub.4 complexes undergo transamination reactions under very mild conditions (J. Chem. Soc. 1960, 3857). Then in 1988, D. Seyferth, and G. Mignani reported that polymeric titanium imides could be transaminated with NH.sub.3 and pyrolized to form titanium nitride in the form of a porous solid ceramic (J. Mater. Sci. Lett. 7, p. 487, 1988). Based on these earlier studies, Renaud M. Fix, Roy G. Gordon, and David M. Hoffman, colleagues at the Department of Chemistry of Harvard University, hypothesized that TiN might be synthesizeable with an APCVD process similar to that devised by Gordon and Kurtz, but using ammonia and Ti(NR.sub.2).sub.4 compounds as precursors. The results of their confirming experiments were presented in a paper delivered at the Material Research Society Symposium (Mat. Res. Soc. Symp. Proc. Vol. 168). Smooth (i.e., mirror-like), nonporous, gold-colored TiN films were produced at temperatures between 100.degree. and 400.degree. C., using ammonia and the metal-organic compound tetrakis(dimethylamido)titanium, Ti(NMe.sub.2).sub.4, as precursors. Flow rate ratios of less than 10:1 of ammonia to Ti(NMe.sub.2).sub.4 were utilized.
A problem with TiN films produced with chemical vapor deposition using Ti(NMe.sub.2).sub.4 and NH.sub.3 as precursors at the aforementioned flow rates is that, in spite of the apparent high quality and golden hue of the deposited TiN films, the resistivity of the films is highly unstable, increasing rather dramatically as a function of the time the film is exposed to the atmosphere. Since TiN films are often used for conductive barrier layers in integrated circuit structures, a TiN film having high resistivity is unsuitable for such uses. Resistive instability of TiN films is demonstrated by depositing a TiN film on a wafer heated to 300.degree. C. in a low-pressure chemical vapor deposition (LPCVD) chamber. Approximately 5 sccm of Ti(NMe.sub.2).sub.4 was introduced into the chamber from a bubbler heated to 50.degree. C. into which a helium carrier gas was introduced at 30 sccm. When ammonia flow was 30 sccm, sheet resistivity of the deposited TiN film was in excess of 1 megohms/square. When ammonia flow was increased to 50 sccm, sheet resistivity of the deposited TiN film was initially approximately 300 kilo-ohms/square. Within two hours, this value had increased by 30 percent, and after 48 hours, the sheet resistivity was in excess of 1 mega-ohms/square. It is postulated that the cause of this instability is the existence of unsaturated titanium bonds in the deposited TiN films. When the newly-created TiN films are exposed to the atmosphere, oxygen most likely diffuses into the films and forms titanium dioxide. Since titanium dioxide (TiO.sub.2) is an exceptionally good dielectric, even a small amount of it will dramatically increase the resistance of a TiN film. SIMS analysis of the deposited TiN films indicates that oxygen content increases from less than 5 percent upon removal from the deposition chamber to as much as 20 percent 48 hours later.
What is needed is a chemical vapor deposition process for TiN which will result in highly conformal films of stable, low resistivity.